Repetition with loops

Loops run a block more than once. A while loop checks its condition before every iteration:

namespace Countdown

function main()
    local number = 3

    while number > 0 do
        print(number)
        number = number - 1
    end

    print("Go!")
end

This program prints 3, 2, 1, and then Go!. When number > 0 becomes false, execution continues after end.

Because the condition is checked first, a while body might run zero times:

local number = 0
while number > 0 do
    print(number) -- never runs
end

As with if, a loop condition must be a Boolean.

Running the body at least once

A repeat loop checks its condition after the body:

local number = 1

repeat
    print(number)
    number = number + 1
until number > 3

The body runs for 1, 2, and 3. Even when the condition is already true in principle, a repeat body always runs once before Pop checks until.

A local declared in the body remains available to the until condition, which is useful when the condition depends on work just performed:

repeat
    local next = calculateNext()
    use(next)
until finished(next)

That local is not available after the loop finishes.

Inclusive numeric ranges

Use numeric for when an integer moves from one bound to another:

for index = 1, 5 do
    print(index)
end

The range includes both bounds. An optional third expression sets the step:

for even = 2, 10, 2 do
    print(even)
end

for countdown = 3, 1, -1 do
    print(countdown)
end

The bounds and step are evaluated once from left to right and must have one identical fixed integer type. The loop binding is body-local and immutable. A zero step is invalid; checked progression can trap on overflow.

Generalized for value in collection iteration is not implemented in rc.3. Arrays can use a numeric range from 1 through Array.length(values); the planned generalized form waits for the nominal Iterable<T> and Iterator<T> protocols.

Leaving or skipping

break exits the innermost loop. continue advances it to its natural condition or range step:

for number = 1, 10 do
    if number == 3 then
        continue
    elseif number > 6 then
        break
    end

    print(number)
end

Both statements work in while, repeat, and numeric for loops. They cannot cross a nested function boundary or appear outside a loop.